1. Field of Invention
This invention relates to electromagnetic relays in which eddy currents induced by electromagnets produce torque to rotate a disc to close a set of electrical contacts. In particular, it relates to such relays having a construction which increases the closing torque upon closure of the contacts to prevent chatter under marginal closing conditions. It has particular application to synchro-verifier relays used in electric power systems to assure that the voltages on two sides of a circuit breaker are approximately the same in magnitude and phase before the breaker is closed.
2. Background Information
A common type of electromagnetic relay includes a rotatable electrically conductive disc carrying a movable contact which makes with a fixed contact when the disc is rotated in a closing direction. Typically, the disc is biased in the open direction by a spring. The disc is rotated in the closing direction against the spring bias by a magnetic element. The magnetic element has an E-shaped core with the ends of the legs of the core forming with a return bar a gap through which the edge of the electrically conductive disc rotates. Magnet flux produced by a current applied to a coil wound on the center leg of the core divides between the two outer legs to form two magnetic circuits which include thegap through which the disc rotates. A shorted lag coil wound on one of the outer legs of the core shifts the phase of the flux in one of the magnet circuits with respect to the other. The out of phase fluxes thus produced in the gap result in application of a closing torque on the disc.
One type of such a relay is a synchro-verifier relay used to determine whether the voltages on the two sides of a circuit breaker are approximately the same in phase and magnitude before the circuit breaker is closed. This relay includes two magnet elements each with an E-shaped magnet core forming a gap through which the disc rotates; an operating element and a restraint element. Each of the magnet elements have two input coils wound on the center leg and a lag coil on one outer leg. The two voltages to be compared are applied to the two input coils of the operating element in the same sense to produce a flux proportional to the sum of the applied voltages, while the same voltages are applied to the two input coils of the restraint element in the opposite sense to produce a flux proportional to the difference between the two voltages.
The fluxes produced in the center legs of the two elements divide between the two outer legs. The lag coil on one outer leg of the core of the operating element shifts the phase of the flux in that leg to produce closing torque on the disc. On the other hand, the lag coil on the one outer leg on the restraint element causes a phase shift which results in the application of an opening torque on the disc. When the two voltages are nearly the same in phase and magnitude, the closing torque will be greater than the opening torque plus a spring bias and the contacts will close. When the voltages are clearly not the same in either/or both phase and magnitude, the opening torque plus the spring bias exceeds the closing torque and the contacts are open. However, when the difference between the two voltages is near the maximum for closure, the closing torque is insufficient to make firm electrical contact and the contacts chatter and do not remain closed. This could occur also in a relay with a single magnetic element when the closing torque is insufficient to hold the contacts closed against the spring bias.
There is a need therefore for an improved electromagnetic relay of this type which makes and maintains solid electrical contact throughout its operating range.
There is a particular need for a synchro-verifier relay which closes without chattering even at the limits of the difference in phase and/or magnitude between two applied voltages. There is a further need for such improvements which can be made easily and economically to existing relays.